Vacuum pumps typically comprise an impeller in the form of a rotor mounted on a rotor shaft for rotation relative to a surrounding stator. The rotor shaft is supported by a bearing arrangement that may comprise two bearings located at or intermediate respective ends of the shaft. One or both of these bearings may be a rolling bearing.
A typical known rolling bearing comprises an inner race fixed relative to the rotor shaft, an outer race and a plurality of rolling elements located between the races for allowing relative rotation of the inner race and the outer race. To prevent mutual contact between the rolling elements, they are often guided and evenly spaced by a cage. Adequate lubrication is essential to ensure accurate and reliable operation of rolling bearings. The main purpose of the lubricant is to establish a load-carrying film separating the bearing components in rolling and sliding contact in order to minimise friction and wear. Other purposes include the prevention of oxidation or corrosion of the bearing components, the formation of a barrier to contaminants and the transfer of heat away from the bearing components.
The lubricant is generally in the form of either oil or grease (a mixture of oil and a thickening agent). However, certain lubricants are not suitable for certain vacuum pump configurations and certain applications on which the vacuum pump is operating. In particular some industrial applications require the vacuum pump to operate without causing any contamination in the chamber that the pump is evacuating. As a result, hydrocarbon oils having a low vapour pressure might be considered as suitable for use within the vacuum pump mechanism. However, the vacuum pump designer also has to consider the chemistry of gases that pass through the pump during evacuation of a chamber. Thus, a lubricant has to be selected that would not react chemically with the gases passing through the pump. Further considerations also need to be given to the operating temperature of the pump. Lubricants have an optimum operating temperature parameter window and some lubricants are known to degrade if they are exposed to operating temperatures above a threshold.
For example, in the manufacture of certain semi-conductor products an ultra-clean environment is required in the process chamber. This can be achieved by utilising so-called dry vacuum pumps where the pump's swept volumes are free from any sealing fluid used to seal the gap between the pump rotor and stator. Such dry vacuum pumps also use ball bearings to facilitate rotation of the rotor shaft. The bearings are also required to operate in an environment of reduced pressure. Thus, synthetic lubricants are used to lubricate the bearings in order to try and prevent contamination of the evacuated chamber that may otherwise occur if certain hydrocarbon lubricants were used to lubricate the bearings. Furthermore, the chemical stability and operating temperature range of certain synthetic lubricants makes this type of lubricant an attractive proposition for use in vacuum pumps. Suitable synthetic lubricants having a suitably chemical inert nature for use in semiconductor processing applications include compounds that are collectively known as perfluoropolyether (PFPE). Additionally, some PFPE compounds are known to have a suitably low vapour pressure, making such compounds suitable for vacuum applications. An example of such a compound is sold under the registered trademark FOMBLIN.